Chemistry 1011

1
Chemistry 1011

• TOPIC
• Gaseous Chemical Equilibrium
• TEXT REFERENCE
• Masterton and Hurley Chapter 12

2
12.5 Effect of Changes in Conditions Upon an
Equilibrium System

• YOU ARE EXPECTED TO BE ABLE TO
• Define Le Chateliers Principle.
• Use Le Chateliers Principle to predict
  qualitatively the effect on an equilibrium system
  of changes in
• concentration (partial pressure) of individual
  components
• total pressure of the system at constant volume
• volume of the system
• total thermal energy of the system
• Predict the effect on an equilibrium of adding a
  catalyst
• Describe industrial processes for the manufacture
  of ammonia and sulfur trioxide

3
Le Chateliers Principle

• A chemical equilibrium can be disturbed by
  changing the external conditions
• Changing the pressure or volume
• Adding or removing a component
• Changing the temperature
• When an external change is made to an equilibrium
  system, the system will alter so as to oppose the
  change

4
Changing the Pressure or Volume

• Changing the pressure or volume of a system will
  result in compression or expansion
• If possible, the system will change and the
  equilibrium will shift so as to oppose the
  compression or expansion
• This can only occur if the total number of moles
  or product is different from the total number of
  moles of reactant

5
Compressing the N2O4 NO2 Equilibrium System

• N2O4(g) 2NO2(g)
• Compressing the equilibrium system by reducing
  the volume will increase the pressure
• The system will shift so as to reduce the
  pressure
• The reverse reaction will take place since this
  results in a decrease in the total number of
  molecules
• 2NO2 (g) N2O4(g)

6
Effect of Pressure on Equilibrium Position

• Compression Expansion
• N2O4(g) ? ?2NO2(g) ?
• N2(g) 3H2(g) 2NH3(g)
• H2(g) I2(g) ? 2HI(g) no effect
  no effect
• N2(g) O2(g) 2NO(g) no effect
  no effect

7
Adding or removing a Gaseous Component

• Adding a gaseous reactant or product to an
  equilibrium system will disturb the equilibrium
• The system will shift so as to remove the added
  species
• Removing a gaseous reactant or product from an
  equilibrium system will disturb the equilibrium
• The system will shift so as to replace the
  removed species

8
Modifying the N2O4 NO2 Equilibrium System by
Adding/Removing Components

• N2O4(g) 2NO2(g)
• Adding more N2O4 - reaction occurs in forward
  direction
• Adding more NO2 - reaction occurs in reverse
  direction
• Removing N2O4 - reaction occurs in reverse
  direction
• Removing NO2 - reaction occurs in forward
  direction

9
Confirming Le Chateliers Principle

• A determination of the reaction quotient
  immediately after adding (or removing) a gaseous
  component will confirm Le Chateliers Principle
• For N2O4(g) 2NO2(g)
• Kp (PNO2)2/PN2O4
• Adding NO2 will raise PNO2 and lower PN2O4
• Q will be gtKp Reverse reaction will occur

10
Changing the Temperature

• Changing the temperature of a system will disturb
  the equilibrium
• The system will change and the equilibrium will
  shift so as to oppose the change in temperature
• If the temperature is raised, the reaction will
  proceed in the endothermic direction until a new
  equilibrium is reached at a higher temperature
• If the temperature is lowered, the reaction will
  proceed in the exothermic direction until a new
  equilibrium is reached at a lower temperature

11
Modifying the N2O4 NO2 Equilibrium System by
Changing the Temperature

• The reaction
• N2O4(g) 2NO2(g) DHo 57.2kJ
• (colourless) (brown)
• is endothermic in the forward direction
• An increase in temperature will cause the forward
  reaction to take place in order to absorb the
  added heat (Le Chatelier)
• A new equilibrium will be established at the
  higher temperature
• PNO2 will be greater PN2O4 will be less
• The gas mixture will become more brown

12
Confirming Le Chateliers Principle

• The vant Hoff equation relates the values of the
  equilibrium constant for a reaction at different
  temperatures to the value of DHo
• ln K2 DHo 1 - 1
• K1 R T1 T2
• If DH is ve, then
• K2 is smaller than K1 if T2 gt T1

13
Effect of Changes in Conditions Upon an
Equilibrium System

• If the number of reactant molecules is different
  from the number of product molecules, changing
  the total pressure at equilibrium will change the
  equilibrium composition. Kp WILL NOT change
• Adding or removing a gaseous reactant or product
  species will change the equilibrium composition.
  Kp WILL NOT change
• Changing the temperature will change the
  equilibrium composition. Kp WILL change

14
Effect of Catalysts on Equilibrium

• Adding a catalyst will not alter the equilibrium
  concentrations of reactants or products. Kp WILL
  NOT change
• Adding a catalyst WILL result in a reaction
  reaching equilibrium more quickly

15
Applying Le Chateliers Principle The Haber
Process

• N2(g) 3H2(g) 2NH3(g) DH -92kJ
• Kp (PNH3)2 6.0 x 105
  at 25oC
• PN2 x (PH2)3
• The number of product molecules is 2, the number
  of reactant molecules is 4
• The forward reaction is exothermic
• The value of Kp decreases as temperature rises
• At 227oC Kp 0.10
• The activation energy for the forward reaction is
  gt150kJ

16
Choosing the Best Conditions

• At 25oC the equilibrium favours NH3, but at 25oC
  the reaction rate is almost zero
• High temperatures are required in order to have a
  reasonable number of reactant molecules with
  energy gt activation energy
• While the rate will increase at higher
  temperatures, the equilibrium yield of ammonia
  will be lower
• Raising the pressure both favours a higher
  equilibrium yield of ammonia and increases the
  rate
• Adding a catalyst will result in a lower
  activation energy

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The Haber Process Compromise

• Moderate temperature 450oC
• High pressure 200 to 600 atm
• Carefully selected catalyst
• Extra nitrogen
• Reactants recycled as ammonia removed from system

18
Applying Le Chateliers Principle The Contact
Process

• Sulfur is burned in air
• S(s) O2(g) SO2(g)
• Sulfur dioxide is reacted with more oxygen
  using a catalyst
• SO2(g) 1/2O2(g) SO3(g) DH -98.9kJ
• Sulfur trioxide is reacted with water
• SO3(g) H2O(l) H2SO4(l)

19
The SO2 - SO3 Equilibrium

• The forward reaction is exothermic higher
  temperatures favour reactants, low temperatures
  preferred
• (at 200oC Kp 1.0 x 106 at 600oC Kp 10)
• Low temperatures result in very low rates - high
  temperatures are required if reactant molecules
  are to overcome the actvation energy barrier
• High pressures favour products and result in
  faster rates

20
The Contact Process Compromise

• Temperature not so high as to favour reactants,
  but high enough to result in rapid rate
• Use of a carefully selected catalyst
• Pass reactant mixture over catalyst beds at
  moderate temperatures 450oC to 600oC
• First pass at high temperature (600oC) results in
  rapid attainment of equilibrium with 80
  conversion of to
• Second pass at results in 99 conversion
• (Note SO3 will not react with water! It must be
  dissolved in concentrated H2SO4. The resulting
  mixture is then diluted)